Many traumatic or pathological cardiac conditions can be corrected, or at least partially compensated for, by providing a pneumatically or hydraulically activated pump to supplement the function of the compromised heart and assist in maintaining normal blood perfusion. Significant advances have been made in recent years in the development of such vascular prosthesis. The main problem encountered by those working in this field concerns the complicated processes occurring at the blood-graft-interface. It has been found that the addition of a foreign material to the blood stream usually results in the formation of a blood clot on the surface of that material, leading to thromboembolic complications and death.
One form of heart-assist system that has been proposed comprises a diaphragm type pump adapted to be surgically implanted and connected in series between the left ventricle and descending aorta. The left ventricle is chosen because it performs approximately 83 percent of the work done by the heart on the blood stream. This type of blood pump is shown schematically in FIG. 1. However, for more complete details of a blood pump of the type shown in FIG. 1, see U.S. Pat. No. 3,604,016 to Robinson et at., entitled MULTIPLE FUNCTION BLOOD COUPLER, the teachings of which are incorporated herein by reference.
As is shown in FIG. 1, the blood pump 10 typically comprises an outer housing 12 of titanium or other suitable metal provided with an outlet passage 14 adapted to be grafted onto the aorta. An inlet passage 16 is designed to be secured to the left ventricle. The pump 10 is provided with inlet and outlet check valves 18, 20 to control the flow to and from the pump. A flexible tube 22 connects the housing 12 to the pump control unit (not shown).
A thin, flexible polyurethane bladder 24 is contained within the housing 12. The elastomer of the present invention is employed to great advantage in forming this bladder 24 (or bladder 51 of the pump shown in U.S. Pat. No. 3,604,016). The bladder 24 wall thickness can vary between 0.020 inch and 0.050 inch, but preferably is between 0.025 and 0.030 inch, since at this thickness the material is still highly flexible, yet retains a safe margin of strength.
The blood contacting surfaces of all parts, including the hardware as well as the polyurethane blood pump itself are preferably coated with a surface which encourages the formation of an intimal lining. This coating can be formed by flocking with a matted coat of dacron fibrils, in the manner known in the art. Although the interior surface of the bladder is customarily flocked with dacron fibrils to encourage formation of an autologous pseudoendothelial surface intraimplant, the polyurethane material of this invention is not necessarily restricted to this configuration. Indeed, all the physical and biological properties of the elastomer of the present invention are obtained from smooth molded films.
As should be apparent, the bladder 24 must be formed of a material that meets a number of criteria. For example, the material from which the bladder 24 is formed must be capable of continuously flexing without breaking. If the bladder is to be used in an artificial heart, the bladder would have to flex continuously without interruption for a minimum of one year. During this period of time, the bladder would undergo about 40,000,000 flexes. In addition to being able to exhibit the ability to flex continuously without breaking, the material from which the bladder is fabricated must be hemo-compatible. That is, the material should not induce the formation of a thrombus which can migrate into the peripheral blood stream; and lastly, the raw materials from which the bladder is made cannot be toxic.
There are several materials on the market which are being used, at least experimentally, as a bladder material. These materials are polyurethane based. However, these known prior art polyurethane based bladder materials suffer from a number of deficiencies. The most significant definciency of these materials is the result of the manufacturing techniques used when employing these polyurethane materials to form a bladder. The prior art polyurethane based bladder materials are used in a solvated liquid. To form a bladder, a form is dipped into the solvated polyurethane liquid and the form is withdrawn. Thereafter, the solvent is evaporated. This procedure is repeated until the polyurethane material has built up to the desired thickness. Thus, a multiple series of dipping and withdrawals is involved in order to obtain a bladder.
In contradistinction to the foregoing procedure, the procedure by which the bladder of the present invention is made involves no solvent. The constituents of the elastomer are 100% non-volatile. The polymer is formed by mixing the constituents together and pouring the constituents into a mold. The desired thickness of the bladder is determined by the thickness of the mold.